Atmospheric Acetaldehyde: Importance of Air‐Sea Exchange and a Missing Source in the Remote Troposphere

Abstract

We report airborne measurements of acetaldehyde (CH3CHO) during the first and second deployments of the National Aeronautics and Space Administration Atmospheric Tomography Mission (ATom). The budget of CH3CHO is examined using the Community Atmospheric Model with chemistry (CAM‐chem), with a newly developed online air‐sea exchange module. The upper limit of the global ocean net emission of CH3CHO is estimated to be 34 Tg/a (42 Tg/a if considering bubble‐mediated transfer), and the ocean impacts on tropospheric CH3CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH3CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid is an ideal indicator of the rapid CH3CHO production in the remote troposphere. The higher‐than‐expected CH3CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry‐climate models.

Global Model for Depth‐Dependent Carbonyl Photochemical Production Rates in Seawater

“…It is proposed that the oceans serve as a net source for atmospheric acetaldehyde through air‐sea exchange; however, fluxes vary widely (Beale et al, 2013; Millet et al, 2010; Singh et al, 2004; Zhu & Kieber, 2019). An annual global sea to air flux of 34–57 Tg year −1 was calculated using an oceanic mixed‐layer photochemical flux model, with remotely sensed chromophoric dissolved organic matter (CDOM) absorption used as model input as a proxy to extrapolate rate data to the oligotrophic ocean (Millet et al, 2010; Wang et al, 2019). This modeled flux is substantially greater than the global sea‐to‐air acetaldehyde flux of ~3–17 Tg year −1 calculated based on in situ observations across the Atlantic Ocean (Beale et al, 2013; Yang et al, 2014).…”

Evidence for an Oceanic Source of Methyl Ethyl Ketone to the Atmosphere

“…Earlier versions of TOGA had difficulty measuring some carbonyls in clean-air conditions (including acetone and acetaldehyde; Apel, 2003); however, neither the original studies nor more recent updates have suggested that there are any issues in measuring MEK. Changes to TOGA have lowered detection limits and increased the capability of the instrument in measuring carbonyl mixing ratios in clean conditions (Apel et al, 2015;Wang et al, 2019). Further discussion of the tests performed regarding these measurement artifacts in preparation for the ATom campaigns can be found in the supplementary material to this work (for MEK and acetone) and in the supplement to Wang et al (2019).…”

“…Changes to TOGA have lowered detection limits and increased the capability of the instrument in measuring carbonyl mixing ratios in clean conditions (Apel et al, 2015;Wang et al, 2019). Further discussion of the tests performed regarding these measurement artifacts in preparation for the ATom campaigns can be found in the supplementary material to this work (for MEK and acetone) and in the supplement to Wang et al (2019). Data from both WAS and TOGA registering below the lower limit of detection (LLOD) were dynamically assigned a value between 0 and the LLOD as a linear function of how many LLOD-flagged values were recorded on a given flight: the more LLOD-flagged non-NA observations, the closer the assigned value was to 0.…”

“…We use a simplified two-layer model developed by Liss and Slater (1974) to calculate the air-sea exchange of MEK. This model, used in many air-sea flux schemes (Fischer et al, 2012;Johnson, 2010;Wang et al, 2019), parameterizes the flux of a gas into or out of the ocean based on windspeeds and concentration gradients. We supplement the original two-layer model with updated air (Johnson, 2010) and water (Nightingale et al, 2000) transfer velocities.…”

“…The production of DMS is so tightly coupled to oceanic productivity that seabirds use it as a cue to find prey in the open ocean (Nevitt et al, 1995). Other VOCs have primarily photochemical sources in the ocean: Acetaldehyde in the surface ocean has been shown to originate from the photolysis of chromophoric dissolved organic matter (CDOM; Kieber et al, 1990;Millet et al, 2010;Wang et al, 2019). VOCs such as methyl nitrate (CH 3 ONO 2 ) have potentially more complicated oceanic sources.…”